Proper functioning of the auditory system requires the integration of incoming sensory stimuli with central feedback information in both the periphery and central auditory centers. Since human cochlear prostheses should incorporate both incoming and descending auditory information, any knowledge regarding the integration of such information should be useful in their design. in this regard, the normal physiology of cochlear hair cells and their ascending projections in the mammalian auditory system have been well characterized; the available data on the descending auditory system, however, are sparse by comparison. For example, relatively little is known about the mechanisms by which olivocochlear (OC) fibers modulate auditory nerve inputs in the cochlea. Similarly, in the cochlear nucleus (CN), the complex interaction between auditory afferents and efferents that results in the modulation of neuronal response properties remains to be elucidated. The avian auditory system might be more useful than the mammalian system in the clarification of the relationship between afferent and efferent inputs because its neuronal architecture and connectivity ace less complex than those in mammals. A first step in understanding the contribution of descending inputs to the normal physiology of the chick auditory system is the neuroanatomical localization of their cells of origin. Identification of efferent neurons and the neurotransmitters that they employ is necessary for elucidating the functional role that efferents play in shaping the response properties of hair cells and auditory neurons. The goals of the proposed investigation are to define the source(s) of efferent projections to the avian cochlea and cochlear nucleus both anatomically and neurochemically. Injections of tritiated gamma-amino butyric acid (GABA) will be made into the chick CN or cochlea and autoradiographic procedures will be used to determine the location of GABAergic cells of origin. Choline acetyltransferase immunocytochemistry and acetylcholinesterase histochemistry will be employed to determine sources of cholinergic input and fiber pathways, respectively, to the cochlea and CN. Additionally, a combination of fluorescent tracers will be used to determine the existence and patterns of axonal collateralization of descending projections.